New unification theories predict large extra dimensions (LEDs).If that is the case,gravity would be stronger at short ranges than what Newtonian gravity predicts.LEDs could also have effects at atomic level.In this paper we propose a new method to constrain the size of ’gravity-only’ LEDs by analysing how these LEDs modify the energy of the atomic transitions 1s-2s and 2s-2p (Lamb shift),in the particular case of the hydrogen and muonium atoms.We estimate these effects by using Bethe’s non-relativistic treatment of Lamb shift.In the particular case of three LEDs,which may be a candidate to explain the interaction mechanism of dark matter particles,we have found that current knowledge in atomic spectroscopy could constrain their sizes to less than 10 μm.Although our contributions do not reach the sensitivity given by SN1987a,they are still slightly better than recent constraints given by Inverse Square Law tests of the E(o)t-Wash group at Washington University,which gave R3＜ 36.6μm.